We studied near-infrared disk fractions of six young clusters in the low-metallicity environments with [O/H] ∼ −0.7 using deep JHK images with Subaru 8.2 m telescope. We found that disk fraction of the low-metallicity clusters declines rapidly in <1 Myr, which is much faster than the ∼5-7 Myr observed for the solar-metallicity clusters, suggesting that disk lifetime shortens with decreasing metallicity possibly with an ∼10 Z dependence. Since the shorter disk lifetime reduces the time available for planet formation, this could be one of the major reasons for the strong planet-metallicity correlation. Although more quantitative observational and theoretical assessments are necessary, our results present the first direct observational evidence that can contribute to explaining the planetmetallicity correlation.

The extreme outer Galaxy (EOG), the region with a Galactic radius of more than 18 kpc, is known to have very low metallicity, about one-tenth that of the solar neighborhood. We obtained deep near-infrared (NIR) images of two very young (∼0.5 Myr) star-forming clusters that are one of the most distant embedded clusters in the EOG. We find that in both clusters the fraction of stars with NIR excess, which originates from the circumstellar dust disk at radii of ≤0.1 AU, is significantly lower than those in the solar neighborhood. Our results suggest that most stars forming in the low-metallicity environment experience disk dispersal at an earlier stage (<1 Myr) than those forming in the solar metallicity environment (as much as ∼5-6 Myr). Such rapid disk dispersal may make the formation of planets difficult, and the shorter disk lifetime with lower metallicity could contribute to the strong metallicity dependence of the well-known "planetmetallicity correlation", which states the probability of a star hosting a planet increases steeply with stellar metallicity. The reason for the rapid disk dispersal could be increase of the mass accretion rate and/or the effective far-ultraviolet photoevaporation due to the low extinction; however, another unknown mechanism for the EOG environment could be contributing significantly.Subject headings: infrared: stars -planetary systems: protoplanetary disks -stars: pre-main-sequence -open clusters and associations: individual (Digel Cloud 2) -stars: formation 1 Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

As a first step for studying star formation in the extreme outer Galaxy (EOG), we obtained deep near-infrared images of two embedded clusters at the northern and southern CO peaks of Cloud 2, which is one of the most distant star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the IRCS imager at the Subaru telescope, we detected cluster members with a mass detection limit of < 0.1 M_{sun}, which is well into the substellar regime. These high quality data enables a comparison of EOG to those in the solar neighborhood on the same basis for the first time. Before interpreting the photometric result, we have first constructed the NIR color-color diagram (dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the Mauna Kea Observatory filter system and also for the low metallicity environment since the metallicity in EOG is much lower than those in the solar neighborhood. The estimated stellar density suggests that an ``isolated type'' star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation is ongoing in Cloud 2-S. Despite the difference of the star formation mode, other characteristics of the two clusters are found to be almost identical: (1) K-band luminosity function (KLF) of the two clusters are quite similar, as is the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the estimated star formation efficiencies (SFEs) for both clusters are typical compared to those of embedded clusters in the solar neighborhood (~ 10 %). The similarity of two independent clusters with a large separation (~ 25 pc) strongly suggest that their star formation activities were triggered by the same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap

We derived the intermediate-mass (≃1.5-7 M ⊙ ) disk fraction (IMDF) in the nearinfrared JHK photometric bands as well as in the mid-infrared (MIR) bands for young clusters in the age range of 0 to ∼10 Myr. From the JHK IMDF, the lifetime of the innermost dust disk (∼0.3 AU; hereafter the K disk) is estimated to be ∼3 Myr, suggesting a stellar mass (M * ) dependence of K-disk lifetime ∝ M −0.7 * . However, from the MIR IMDF, the lifetime of the inner disk (∼5 AU; hereafter the MIR disk) is estimated to be ∼6.5 Myr, suggesting a very weak stellar mass dependence (∝ M −0.2 * ). The much shorter K-disk lifetime compared to the MIR-disk lifetime for intermediatemass (IM) stars suggests that IM stars with transition disks, which have only MIR excess emission but no K-band excess emission, are more common than classical Herbig Ae/Be stars, which exhibit both. We suggest that this prominent early disappearance of the K disk for IM stars is due to dust settling/growth in the protoplanetary disk, and it could be one of the major reasons for the paucity of close-in planets around IM stars.